Does anyone know the maximum tension a MP 623 can withstand? - and the original tension with which it is strung?
I don't know. But i am currently using 27 on my MP623. I have 3 of MP 623 all using tension 27. So far, no problem.
22lbs is normal maximum string tension for Yonex racquets, it should be safe over 22lbs but the stringer/yonex are unlikely to replace it if it breaks at higher tension.